**3. Crop residue and harvest management**

increased in comparison with those observed in the treatment that included only mineral fertilizer. Cumulatively, the highest emissions were observed for ratoon sugarcane treated with vinasse, especially as the amount of crop residue on the soil surface increased. Normally, the

this GHG [53]. In general, filter cakes can be associated with a lower emission factor compared with other organic or synthetic fertilizers [25]. In turn, the vinasse application can increase

O emissions from sugarcane soils, especially during the first couple of days after application [26, 53]. The applied vinasse generates a high emission factor analogous to the emission

Another organic fertilizer is sewage sludge. Although sewage sludge is also very lacking in K, it has high levels of P [60]. This organic fertilizer can improve soil's physical and chemical characteristics and can increase sugarcane productivity, acid phosphatase activity, and biomass [61]. These authors also highlighted the beneficial effect of B, Zn, and Cu from sewage sludge in association with available P that provided increase in the stalks production. However, its use requires some care, as there is the possibility of pathogen and heavy metal contamination. The application of sewage sludge may increase the concentrations of As, Cd, Cu, Ni, Pb, and Zn in the soil, and the quality standard established by the legislation for agricultural soils must be respected [62]. However, the incorporation into soils of sewage sludge rich in C has been shown to increase the amount of dissolved organic matter in soils. Dissolved organic matter can facilitate metal transport in soil through formation of soluble metal-organic complexes [63, 64]; in contrast, they are also able to mobilize some heavy metals sorbed from soil or sewage sludge, being the soil organic matter one of the most important solid phases that adsorb heavy metals, such as Cu and Cd in acid sandy soils. Thus, soils amended with sewage sludge display different physicochemical properties, especially in terms of dissolved organic matter in soil, which will affect behavior of metals in soils.

However, the impact of sewage sludge in the environment on the soil microbial community

The incorporation of ecological practices into sugarcane production and management has the potential to arrest and ameliorate the negative effects of monocropping on soil degradation and yield decline. Historically, the production of green manure as a cover or break crop has been shown to improve the physical, chemical, and biological characteristics of the soil for many crops in production agriculture. Schumann et al. [66] published an interesting review of green manuring practices in sugarcane production. However, only recently, the effects of green manure on soil microbial populations, diversity, and activity in sugarcane soils have been reported [67], in which decrease in the total bacterial population in the soil was revealed, while that of fungi and actinomycetes increased. In addition, Ambrosano et al. [68] verified that green manure is an alternative source of N for sugarcane crops and can supplement or even replace mineral N fertilization. Moreover, green manure associated with mineral N fertilizer altered the soil chemical factors, increasing Ca and Mg contents, sum of bases, soil pH and base saturation, and as a consequence decreased the

The application of sewage sludge can also provide an increase in CO<sup>2</sup>

has not yet been reported for sugarcane agriculture.

potential acidity.

is variable, indicating the ability of the soil to serve either as source or as sink of

emissions in soils [65].

flow of CH<sup>4</sup>

12 Sugarcane - Technology and Research

factor observed for urea application.

N2

Soil residue management focusing in soil quality (conservation) and its energetic use are emerging study subjects regarding the sugarcane crop worldwide. In areas under sugarcane cultivation, different sugarcane harvest systems are commonly applied, such as manual handling with burnt sugarcane (burnt harvest) and mechanical harvesting (green harvest). In Brazil, the world's largest producer of sugarcane, harvest practices for sugarcane are undergoing a change, with the increased introduction of mechanical harvesting. This change is regulated by state legislation. For instance, the states of São Paulo and Goiás, which produce more than half of the sugarcane in Brazil, have similar deadlines to completely change their harvest systems. In these states, sugarcane burning is scheduled to be completely phased out progressively during the next 15 years, depending mainly on land declivity due to mechanization limitations.

Without burning, in average, 8–30 Mg ha−1 dry mass of straw is generated [9, 71, 72], which has 54% dry leaves and 46% tops [73]. The average crop residue produced every year is approximately 10 Mg ha−1 of material with a C:N ratio of approximately 100 [74], that reflects the presence of lignocellulosic composition in the straw, which accounts for 19–34% lignin, 29–44% cellulose, and 27–31% hemicelluloses [75–79]. This characteristic implies in high recalcitrance of residues, that has slow decomposition rate on soil. Around 30–60% of soil moisture content is kept after harvest [80, 81]. There is discussion regarding the feasibility of sugarcane biomass utilization in the industry versus keeping it in the field to improve soil quality and guarantee the long-term sustainability.

Both practices in sugarcane harvest, i.e., burnt and green harvests, have the potential to influence soil physicochemical, microbiological factors, as well as, soil organic fractions. Sugarcane burning as a preharvesting method is a millenary technique to eliminate all leaves and tops around the sugarcane plant, which helps with manual harvest [82] and transport [83]. However, it is known that burnt harvest has the potential to negatively alter the physical, chemical, and biological soil characteristics [21, 84], to increase GHG emissions [85–87], and to decrease soil organic matter [88]. Moreover, particulate matter and smoke from leaf burning released into the atmosphere represent health hazards [89].

In contrast, the maintenance of sugarcane plant residue as a surface blanket positively affects the physical, chemical, and biological soil characteristics. However, these positive effects cannot be observed if soil tillage operations are considered [28]. Conservation agricultural systems, such as minimal soil disturbance (reduced tillage or no tillage), have been sought as an option to conventional tillage practices in order to reduce production costs and improve the soil fertility status [90]. According to Rachid et al. [91], there are no effects from different levels of sugarcane plant residue on the soil bacterial community. However, the authors reported that the soil fungal community can be impacted, and after 12 months, the community can present different structures among the different levels of sugarcane plant residue blankets. Although the physical and chemical characteristics are important for soil quality and sustainability, microorganisms are the main drivers of the nutrient turnover processes in the soil [16] and of the regulation of many atmospheric constituents, such as GHG. In addition, soil microbes have shown many responses to abiotic soil factors, which are clearly affected by microenvironmental changes [14, 92–94].

similar bacterial communities in green sugarcane and native forest soils, while the bacterial community from burnt sugarcane soil was most distinct from the others. *Acidobacteria* and *Alphaproteobacteria* were the most abundant bacterial phylum and class, respectively, across the different soils, with *Acidobacteria* Gp1 accounting for a higher abundance in green sugarcane and native forest soils than in burnt sugarcane soils. In turn, *Acidobacteria* Gp4 abun-

Multi-Analytical Interactions in Support of Sugarcane Agroecosystems Sustainability in Tropical…

In burnt harvest systems, C, N, and S from sugarcane plants volatilize, although they could return to the soil [12]. However, there is an overall tendency of the burnt straw to decrease soil fertility in the long term. The fertilization associated with burnt straw induced by 59 years in Africa [12] and by 35 years in Brazil [95] resulted in decrease of P, K, cation exchangeable capacity, and decrease in Ca and Mg content. In addition, the soil becomes physically exposed due to decreasing of soil organic matter [96] that has great function to binding polysaccharides, fungal hyphae, and humic substances with soil mineral particles forming the soil aggregates [97] and increasing the availability of nutrients [96], which accelerates the loss of chemical fertility [98]. In addition, the harvest burnt also decreases the stability of aggregate

Concerning GHG emissions, Figueiredo and La Scala Jr. [86] reported that burnt harvesting

tem. However, the authors emphasized that fertilizer application to the soil can also influence GHG emissions. Azevedo et al. [99] reported that burnt sugarcane harvesting intensifies CO<sup>2</sup> and carbon monoxide (CO) emissions. Macedo et al. [100] reported emissions of 6.5 kg CH<sup>4</sup> ha−1 in sugarcane burning. With increasing introduction of mechanical harvesting, a reduction

São Paulo between 1990 and 2009 [101]. According to Capaz et al. [101], there is an increase on ozone and CO content during the sugarcane harvest season due to the burning technique. In synthesis, comparing both harvest management systems, the burnt harvest system presents higher GHG emissions, which range from 558.5 kg Ceq ha−1 y−1 to 2209.2 kg Ceq ha−1 y−1 more

Green harvest has become a recommended approach for sugarcane harvesting*.* Studies have shown that the soil microbial community is more abundant, active, and diverse in green sugarcane soil than in burnt sugarcane soil [103–105], which influences positively on the soil physicochemical factors. According to Graham et al. [103], the microbial metabolic quotient decreases with increasing soil depth, with significant increases in microbial-C biomass up to 30 cm of soil depth. In addition, microbial-C biomass was significantly higher in rows than in between rows as well as the bulk density was decreased since the green harvest to foster the

The light fraction from organic matter is another soil quality management parameter that has a chemical composition comparable to that of plant materials [106] and thus, it may be affected by fluctuations in different management practices. Although it represents a small proportion of total soil mass, it contains a significant part of the total soil C and N, so that

eq. ha−1 y−1 compared with the green harvest sys-

http://dx.doi.org/10.5772/intechopen.71180

15

eq. ha−1) of GHG emissions was estimated in the state of

dance was higher in burnt sugarcane soils than in other soils.

on soil surface [10, 12].

increased GHG emissions by 1484.0 kg CO<sup>2</sup>

than that produced by the green harvest system [102].

of 39.3% (from 1.053 to 0.639 t CO<sup>2</sup>

**3.2. Green harvest management**

increase of soil C status [104].

The current main information related to the impact of sugarcane harvest management on the soil microbial community, soil physicochemical factors, including labile organic C fractions, and GHG emissions at multiple scales are reported below, taking into account the development of more sustainable sugarcane productions systems.

#### **3.1. Burnt harvest management**

Sugarcane burning has been used for many years on sugarcane crops, and it is still being used currently. Given that soil microbes represent the majority of biodiversity in terrestrial ecosystems and are intimately involved in key ecosystem functions, such as soil fertility, increased attention has recently been paid to microbial communities present in soils under burnt and unburnt sugarcane. According to Souza et al. [13], the level of microbial-C biomass in the soil is lower in burnt sugarcane systems than in sugarcane harvesting without burning. The authors suggested that microbial-C biomass is a reliable indicator of soil quality for monitoring soils under different sugarcane harvesting systems. In turn, Rachid et al. [15] used a molecular approach to evaluate the effect of sugarcane burning and green harvest methods on the soil microbes in the Brazilian Cerrado, and they showed significant differences on the soil bacterial community and its structure between burnt and green harvest systems, with the *Firmicutes* phylum and *Acidobacteria* classes being the groups most affected by sugarcane burning. In general, significant structural changes of the community were observed, with the burnt harvest management having a greater impact than green harvest management on the native Cerrado soil communities. The authors concluded that due to the great variability of the Cerrado ecosystem, further research is required to confirm these findings with soil samples from different sites and seasons in order to address the impact due to changes in management over the years. Val-Moraes et al. [21] also used a molecular approach to evaluate the effect of sugarcane burning and green harvest methods on the soil microbes, and they showed that liming in the sugarcane burnt system and that green harvest practices affect the soil bacterial community. The authors revealed higher bacterial diversity in sugarcane soils than in native forest soil, with burnt sugarcane soil accounting for a higher richness of unique operational taxonomic units (OTUs) than native forest soil. The authors also observed similar bacterial communities in green sugarcane and native forest soils, while the bacterial community from burnt sugarcane soil was most distinct from the others. *Acidobacteria* and *Alphaproteobacteria* were the most abundant bacterial phylum and class, respectively, across the different soils, with *Acidobacteria* Gp1 accounting for a higher abundance in green sugarcane and native forest soils than in burnt sugarcane soils. In turn, *Acidobacteria* Gp4 abundance was higher in burnt sugarcane soils than in other soils.

In burnt harvest systems, C, N, and S from sugarcane plants volatilize, although they could return to the soil [12]. However, there is an overall tendency of the burnt straw to decrease soil fertility in the long term. The fertilization associated with burnt straw induced by 59 years in Africa [12] and by 35 years in Brazil [95] resulted in decrease of P, K, cation exchangeable capacity, and decrease in Ca and Mg content. In addition, the soil becomes physically exposed due to decreasing of soil organic matter [96] that has great function to binding polysaccharides, fungal hyphae, and humic substances with soil mineral particles forming the soil aggregates [97] and increasing the availability of nutrients [96], which accelerates the loss of chemical fertility [98]. In addition, the harvest burnt also decreases the stability of aggregate on soil surface [10, 12].

Concerning GHG emissions, Figueiredo and La Scala Jr. [86] reported that burnt harvesting increased GHG emissions by 1484.0 kg CO<sup>2</sup> eq. ha−1 y−1 compared with the green harvest system. However, the authors emphasized that fertilizer application to the soil can also influence GHG emissions. Azevedo et al. [99] reported that burnt sugarcane harvesting intensifies CO<sup>2</sup> and carbon monoxide (CO) emissions. Macedo et al. [100] reported emissions of 6.5 kg CH<sup>4</sup> ha−1 in sugarcane burning. With increasing introduction of mechanical harvesting, a reduction of 39.3% (from 1.053 to 0.639 t CO<sup>2</sup> eq. ha−1) of GHG emissions was estimated in the state of São Paulo between 1990 and 2009 [101]. According to Capaz et al. [101], there is an increase on ozone and CO content during the sugarcane harvest season due to the burning technique. In synthesis, comparing both harvest management systems, the burnt harvest system presents higher GHG emissions, which range from 558.5 kg Ceq ha−1 y−1 to 2209.2 kg Ceq ha−1 y−1 more than that produced by the green harvest system [102].

#### **3.2. Green harvest management**

In contrast, the maintenance of sugarcane plant residue as a surface blanket positively affects the physical, chemical, and biological soil characteristics. However, these positive effects cannot be observed if soil tillage operations are considered [28]. Conservation agricultural systems, such as minimal soil disturbance (reduced tillage or no tillage), have been sought as an option to conventional tillage practices in order to reduce production costs and improve the soil fertility status [90]. According to Rachid et al. [91], there are no effects from different levels of sugarcane plant residue on the soil bacterial community. However, the authors reported that the soil fungal community can be impacted, and after 12 months, the community can present different structures among the different levels of sugarcane plant residue blankets. Although the physical and chemical characteristics are important for soil quality and sustainability, microorganisms are the main drivers of the nutrient turnover processes in the soil [16] and of the regulation of many atmospheric constituents, such as GHG. In addition, soil microbes have shown many responses to abiotic soil factors, which are clearly affected by

The current main information related to the impact of sugarcane harvest management on the soil microbial community, soil physicochemical factors, including labile organic C fractions, and GHG emissions at multiple scales are reported below, taking into account the develop-

Sugarcane burning has been used for many years on sugarcane crops, and it is still being used currently. Given that soil microbes represent the majority of biodiversity in terrestrial ecosystems and are intimately involved in key ecosystem functions, such as soil fertility, increased attention has recently been paid to microbial communities present in soils under burnt and unburnt sugarcane. According to Souza et al. [13], the level of microbial-C biomass in the soil is lower in burnt sugarcane systems than in sugarcane harvesting without burning. The authors suggested that microbial-C biomass is a reliable indicator of soil quality for monitoring soils under different sugarcane harvesting systems. In turn, Rachid et al. [15] used a molecular approach to evaluate the effect of sugarcane burning and green harvest methods on the soil microbes in the Brazilian Cerrado, and they showed significant differences on the soil bacterial community and its structure between burnt and green harvest systems, with the *Firmicutes* phylum and *Acidobacteria* classes being the groups most affected by sugarcane burning. In general, significant structural changes of the community were observed, with the burnt harvest management having a greater impact than green harvest management on the native Cerrado soil communities. The authors concluded that due to the great variability of the Cerrado ecosystem, further research is required to confirm these findings with soil samples from different sites and seasons in order to address the impact due to changes in management over the years. Val-Moraes et al. [21] also used a molecular approach to evaluate the effect of sugarcane burning and green harvest methods on the soil microbes, and they showed that liming in the sugarcane burnt system and that green harvest practices affect the soil bacterial community. The authors revealed higher bacterial diversity in sugarcane soils than in native forest soil, with burnt sugarcane soil accounting for a higher richness of unique operational taxonomic units (OTUs) than native forest soil. The authors also observed

microenvironmental changes [14, 92–94].

**3.1. Burnt harvest management**

14 Sugarcane - Technology and Research

ment of more sustainable sugarcane productions systems.

Green harvest has become a recommended approach for sugarcane harvesting*.* Studies have shown that the soil microbial community is more abundant, active, and diverse in green sugarcane soil than in burnt sugarcane soil [103–105], which influences positively on the soil physicochemical factors. According to Graham et al. [103], the microbial metabolic quotient decreases with increasing soil depth, with significant increases in microbial-C biomass up to 30 cm of soil depth. In addition, microbial-C biomass was significantly higher in rows than in between rows as well as the bulk density was decreased since the green harvest to foster the increase of soil C status [104].

The light fraction from organic matter is another soil quality management parameter that has a chemical composition comparable to that of plant materials [106] and thus, it may be affected by fluctuations in different management practices. Although it represents a small proportion of total soil mass, it contains a significant part of the total soil C and N, so that its evaluation can provide an early indication of changes in land use and soil management [107]. Brandani et al. [108] verified that burnt harvest combined with organic management was a strategy for long-term storage of total C and N in the light organic fraction, which were related to the quality (diversity) and quantity (frequency) of organic residue addition [107].

oxide emissions of 420 kg CO<sup>2</sup>

Concerning N<sup>2</sup>

important to the N<sup>2</sup>

CO<sup>2</sup>

and N<sup>2</sup> O.

CO<sup>2</sup>

decreasing N fertilizer inputs [12].

emissions increase from 466 kg CO<sup>2</sup>

Green harvest results in a total CO<sup>2</sup>

harvest system and 247% more N<sup>2</sup>

According to Panosso et al. [112], CO<sup>2</sup>

eq. ha−1 y−1 in a 6-year crop cycle [116].

cane, and the authors reported meaningful CH<sup>4</sup>

However, Acreche et al. [118] reported 43% more CO<sup>2</sup>

eq. ha−1 were estimated when the total N in crop residue and

O emissions is expected

17

http://dx.doi.org/10.5772/intechopen.71180

O emissions from sugarcane soils, despite

O emissions are still unclear in sugar-

O emissions from treatments

eq. ha−1 y−1 during harvest

eq. ha−1 y−1 [99, 100].

emissions from tillering in the green

emissions rates compared with those of CO<sup>2</sup>

eq. ha−1 y−1 (in burnt sugarcane) to approximately 750 kg

O emissions from post-fertilization than in burnt sugar-

emissions were 32% greater in burnt sugarcane, even 7

default values were considered [100]. Because of the high C:N ratio of sugarcane residue, which can range from 70:1 to 120:1 [22], the soil N immobilization should occur in the first phase of straw decomposition. Nevertheless, because gradual availability of others macro

[116]. Fortes et al. [90] observed in a long-term study developed on an Oxisol, that the amounts of straw nutrients released to the soil-plant system (in kg ha−1 and in percentage of initial content) were of 12.7 (31%) of N, 0.7 (23%) of P, 43.1 (92%) of K, 18.2 (54%) of Ca,

that the highest gas fluxes were verified in the treatments with more residue accumulation

without or with 15 Mg ha−1 of sugarcane residue on the soil surface. Nitrous oxide fluxes seem to be higher when crop residue is combined with inorganic N [20, 26, 33]. However, only small areas in sugarcane fields receive inorganic fertilizer, while the majority of the field is

In the first years after conversion from burnt to green harvest, the N fertilizer dose applied to green sugarcane is approximately 30% higher than in burnt sugarcane, increasing GHG emissions by 27% in comparison with burnt sugarcane [99, 100]. Over the years, more crop residue is added to the system, increasing the quantity of readily decomposable organic matter and

GHG emissions due to fossil fuel consumption of green harvest are related to the diesel use in sugarcane agricultural devices and trucks during the mechanical harvest and stalk trans-

operation, with a mean diesel consumption of 74 L ha−1 y−1 for a 5-year crop cycle [85, 86]. Considering diesel consumption during extraction, processing, and distribution, the GHG

Although green harvest showed high GHG emissions due N fertilizer application and fossil fuel consumption, in the first years of the conversion, reduction in the emissions is expected.

years after converting to a green harvest system. In the first years after conversion from burnt to green harvest, Figueiredo and La Scala Jr. [85, 86] reported emission reductions of 310.7 kg

eq. ha−1 y−1, excluding soil carbon sequestration resulting from the crop residue retention.

sequestration of 1173.3 kg CO<sup>2</sup>

O emission, Pitombo [26] showed that amounts of crop residue from 0 to

Multi-Analytical Interactions in Support of Sugarcane Agroecosystems Sustainability in Tropical…

and micronutrients from straw decomposition a decrease in N<sup>2</sup>

8 (70%) of Mg, and 4.6 (65%) of S, after the three crop cycles.

cane soils. Siqueira Neto et al*.* [25] did not find differences in N<sup>2</sup>

portation [117] (**Figure 1**). They account for nearly 300 kg CO<sup>2</sup>

11.3 Mg ha−1 progressively reduced annual N<sup>2</sup>

(**Table 1**). Nevertheless, the effects of crop residue on N<sup>2</sup>

O balance [26].

Based on a molecular fingerprinting approach, Wallis et al. [109] showed distinct bacterial communities in sugarcane soil under a crop residue blanket in a burnt harvest system. In turn, Rachid et al. [84] reported effects of sugarcane green and burnt harvest management on soil bacterial communities and microbial functional genes. The authors revealed that changes in the soil bacterial community were related to harvest management systems, while soil fungal communities were more sensitive to changes in the crop residue retention levels, probably due to the use of the crop residue as a substrate [91]. Regarding the microbial functional genes, changes in the community structure of ammonia-oxidizing bacteria (*amo*A gene) were correlated with the C:N ratio in the soil, while no significant correlations were revealed between the denitrifying bacteria community structure (*nir*K gene) and the analyzed soil chemical factors.

As mentioned above, the main characteristic of the transition from burning sugarcane to green harvest is the retention of sugarcane plant residue on the soil surface [11, 12]. The sugarcane plant residue retention is an effective practice to: (i) reduce infiltration and soil loss rates [110]; (ii) protect the soil surface from high temperature ranges [110–112]; (iii) maintain the soil moisture levels [110, 113]; (iv) increase earthworm populations and soil microbial biomass [110], which are responsible for organic matter decomposition [95, 113], increasing carbon stocks in the 0–10-cm topsoil layer [83]; (v) increase soil stability and help spread micro and macroaggregates in the soil, which are important for maintaining the soil microbial diversity through the conservation of their microhabitats [12, 98]; and (vi) reduce the necessity of weed control [110]. Hence, green harvest can improve the soil structure and increase sugarcane yield [104, 110] and decrease soil erosion losses [10].

Studies have shown that green harvest may be related to decreases in soil porosity and increases in soil compaction as a consequence of the traffic from harvesters [114], being therefore limited with regard improvement of soil physical factors such as soil bulk density and penetration resistance [98], which could influence negatively on the initial development of root systems, as well as the nutrient availability for plants. However, increases in soil organic matter content and improvements in soil aggregation can gradually reduce the soil compaction [110]. Due to the trend for equilibrium in soil organic matter accumulation, deep drainage and increased soil moisture can promote N losses and denitrification even at low rates [113]. However, the increase in soil carbon by crop residue retention during the ratoon cycles can be lost during tillage operations during the sugarcane replanting period [87], inducing similar soil carbon concentrations for burnt and green sugarcane systems [113].

Nutrient recycling is one of the main reasons for maintaining straw in the field [105]. However, in the first year of sugarcane production, only approximately 20% of the crop residue is available for mineralization and then for denitrification and nitrification, resulting in N<sup>2</sup> O emissions from sugarcane plant residues of 71.61 kg CO<sup>2</sup> eq. ha−1 y−1 [115]. Nitrous oxide emissions of 420 kg CO<sup>2</sup> eq. ha−1 were estimated when the total N in crop residue and default values were considered [100]. Because of the high C:N ratio of sugarcane residue, which can range from 70:1 to 120:1 [22], the soil N immobilization should occur in the first phase of straw decomposition. Nevertheless, because gradual availability of others macro and micronutrients from straw decomposition a decrease in N<sup>2</sup> O emissions is expected [116]. Fortes et al. [90] observed in a long-term study developed on an Oxisol, that the amounts of straw nutrients released to the soil-plant system (in kg ha−1 and in percentage of initial content) were of 12.7 (31%) of N, 0.7 (23%) of P, 43.1 (92%) of K, 18.2 (54%) of Ca, 8 (70%) of Mg, and 4.6 (65%) of S, after the three crop cycles.

its evaluation can provide an early indication of changes in land use and soil management [107]. Brandani et al. [108] verified that burnt harvest combined with organic management was a strategy for long-term storage of total C and N in the light organic fraction, which were related to the quality (diversity) and quantity (frequency) of organic residue addition [107]. Based on a molecular fingerprinting approach, Wallis et al. [109] showed distinct bacterial communities in sugarcane soil under a crop residue blanket in a burnt harvest system. In turn, Rachid et al. [84] reported effects of sugarcane green and burnt harvest management on soil bacterial communities and microbial functional genes. The authors revealed that changes in the soil bacterial community were related to harvest management systems, while soil fungal communities were more sensitive to changes in the crop residue retention levels, probably due to the use of the crop residue as a substrate [91]. Regarding the microbial functional genes, changes in the community structure of ammonia-oxidizing bacteria (*amo*A gene) were correlated with the C:N ratio in the soil, while no significant correlations were revealed between the denitrifying bacteria community structure (*nir*K gene) and the analyzed

As mentioned above, the main characteristic of the transition from burning sugarcane to green harvest is the retention of sugarcane plant residue on the soil surface [11, 12]. The sugarcane plant residue retention is an effective practice to: (i) reduce infiltration and soil loss rates [110]; (ii) protect the soil surface from high temperature ranges [110–112]; (iii) maintain the soil moisture levels [110, 113]; (iv) increase earthworm populations and soil microbial biomass [110], which are responsible for organic matter decomposition [95, 113], increasing carbon stocks in the 0–10-cm topsoil layer [83]; (v) increase soil stability and help spread micro and macroaggregates in the soil, which are important for maintaining the soil microbial diversity through the conservation of their microhabitats [12, 98]; and (vi) reduce the necessity of weed control [110]. Hence, green harvest can improve the soil structure and increase sugarcane

Studies have shown that green harvest may be related to decreases in soil porosity and increases in soil compaction as a consequence of the traffic from harvesters [114], being therefore limited with regard improvement of soil physical factors such as soil bulk density and penetration resistance [98], which could influence negatively on the initial development of root systems, as well as the nutrient availability for plants. However, increases in soil organic matter content and improvements in soil aggregation can gradually reduce the soil compaction [110]. Due to the trend for equilibrium in soil organic matter accumulation, deep drainage and increased soil moisture can promote N losses and denitrification even at low rates [113]. However, the increase in soil carbon by crop residue retention during the ratoon cycles can be lost during tillage operations during the sugarcane replanting period [87], inducing similar soil carbon concentrations for burnt and green sugarcane

Nutrient recycling is one of the main reasons for maintaining straw in the field [105]. However, in the first year of sugarcane production, only approximately 20% of the crop residue is available for mineralization and then for denitrification and nitrification, resulting

eq. ha−1 y−1 [115]. Nitrous

O emissions from sugarcane plant residues of 71.61 kg CO<sup>2</sup>

soil chemical factors.

16 Sugarcane - Technology and Research

systems [113].

in N<sup>2</sup>

yield [104, 110] and decrease soil erosion losses [10].

Concerning N<sup>2</sup> O emission, Pitombo [26] showed that amounts of crop residue from 0 to 11.3 Mg ha−1 progressively reduced annual N<sup>2</sup> O emissions from sugarcane soils, despite that the highest gas fluxes were verified in the treatments with more residue accumulation (**Table 1**). Nevertheless, the effects of crop residue on N<sup>2</sup> O emissions are still unclear in sugarcane soils. Siqueira Neto et al*.* [25] did not find differences in N<sup>2</sup> O emissions from treatments without or with 15 Mg ha−1 of sugarcane residue on the soil surface. Nitrous oxide fluxes seem to be higher when crop residue is combined with inorganic N [20, 26, 33]. However, only small areas in sugarcane fields receive inorganic fertilizer, while the majority of the field is important to the N<sup>2</sup> O balance [26].

In the first years after conversion from burnt to green harvest, the N fertilizer dose applied to green sugarcane is approximately 30% higher than in burnt sugarcane, increasing GHG emissions by 27% in comparison with burnt sugarcane [99, 100]. Over the years, more crop residue is added to the system, increasing the quantity of readily decomposable organic matter and decreasing N fertilizer inputs [12].

GHG emissions due to fossil fuel consumption of green harvest are related to the diesel use in sugarcane agricultural devices and trucks during the mechanical harvest and stalk transportation [117] (**Figure 1**). They account for nearly 300 kg CO<sup>2</sup> eq. ha−1 y−1 during harvest operation, with a mean diesel consumption of 74 L ha−1 y−1 for a 5-year crop cycle [85, 86]. Considering diesel consumption during extraction, processing, and distribution, the GHG emissions increase from 466 kg CO<sup>2</sup> eq. ha−1 y−1 (in burnt sugarcane) to approximately 750 kg CO<sup>2</sup> eq. ha−1 y−1 in a 6-year crop cycle [116].

Green harvest results in a total CO<sup>2</sup> sequestration of 1173.3 kg CO<sup>2</sup> eq. ha−1 y−1 [99, 100]. However, Acreche et al. [118] reported 43% more CO<sup>2</sup> emissions from tillering in the green harvest system and 247% more N<sup>2</sup> O emissions from post-fertilization than in burnt sugarcane, and the authors reported meaningful CH<sup>4</sup> emissions rates compared with those of CO<sup>2</sup> and N<sup>2</sup> O.

Although green harvest showed high GHG emissions due N fertilizer application and fossil fuel consumption, in the first years of the conversion, reduction in the emissions is expected. According to Panosso et al. [112], CO<sup>2</sup> emissions were 32% greater in burnt sugarcane, even 7 years after converting to a green harvest system. In the first years after conversion from burnt to green harvest, Figueiredo and La Scala Jr. [85, 86] reported emission reductions of 310.7 kg CO<sup>2</sup> eq. ha−1 y−1, excluding soil carbon sequestration resulting from the crop residue retention.
